We employed recombinant DNA techniques to investigate the molecular biology of dengue virus with the intent of developing immunoprophylactic measures against this virus that is epidemic in many geographic areas. DNA sequences (approximately 11,000 nucleotides) representing the full-length genome of the dengue virus type 4 were cloned. The sequence of the first 2,429 nucleotides at the 5'-terminus, which includes the coding region for the structural proteins, has been determined. The virion structural proteins are encoded in one long open reading frame specifying a polyprotein precursor which is apparently proteolytically cleaved by a mechanism resembling that proposed for expression of structural proteins of other flaviviruses such as yellow fever (YF) and West Nile (WN) viruses. The N-terminus for each of the dengue virus structural proteins was tentatively assigned by homology alignment to the corresponding sequence of YF and WN virus. Comparison of sequence homology of structural proteins suggests that dengue virus is more closely related to WN virus than to YF virus or Murray Valley encephalitis virus. Finally, analysis of the extreme 5'- and 3'-terminal nucleotides of the dengue virus genome revealed sequences that may be involved in transcription, replication and packaging of viral RNA. Attempts are currently underway to determine if cloned full-length dengue DNA is infectious after introduction into permissive cells. If successful, it will then be possible to construct defined viral mutants that may be useful for biological studies. Such mutants may also prove to be useful in the development of a safe, effective live attenuated vaccine for prevention of dengue disease.